Mobile communication systems are provided with a mobile switching center (MSC), a base station controller (BSC), a base transceiver station (BTS), a mobile station (MS), and the like. A communication possible area can be extended where multiple BTSs are installed. However, when a signal transmitted from the BTS is propagated over air, the signal may be weakened. Since signal strength may be further weakened due to natural and artificial obstacles such as a mountain, a building, a building underground, a tunnel, the inside of a building, and the like, there may occur a partial communication shadow zone where smooth reception of the MS is impossible. To address a problem of the communication shadow zone, a repeater is conventionally installed and used.
In mobile communication and broadcast networks, repeaters are widely used for coverage extension and electronic wave shadow zone reduction. Among the repeaters, particularly, a radio frequency (RF) repeater is inexpensive since a signal received from a base station has the same frequency as that transmitted from a mobile station. The RF repeater may be the most preferred repeater on a wireless basis since a special transmission line does not need to be constructed.
As illustrated in FIG. 1, the conventional RF repeater is provided with a donor antenna 10 for communicating with a base station 1, a service antenna 12 for communicating with a mobile station 2, and a repeater 14 for filtering and amplifying a signal between the two antennas. When isolation between the two antennas is not ensured, a signal retransmitted through the service antenna 12 after amplification is fed back to the donor antenna 10 and therefore an amplifier may oscillate. Thus, a scheme is used in which the isolation between the two antennas is maximally ensured (at 60˜70 dB in a conventional case) and amplification gain is set in a range in which a power amplifier does not oscillate.
Since the oscillation of the repeater is vital to a network and a system, it is set that the amplifier gain has a higher margin of 15˜20 dB than the conventional isolation. Thus, the amplifier gain is about 40˜55 dB. This limits a primary function of the repeater, that is, a sufficient coverage extension or electronic wave shadow zone reduction function and is also the largest drawback of the RF repeater.
To obtain the amplifier gain of about 80 dB with sufficient utility in the RF repeater, an isolation value between the two antennas should be set to about 100 dB or more. It is difficult to satisfy the value in an actual situation.
To conventionally ensure the isolation between the two antennas in the RF repeater, there can be used an antenna for ensuring a high isolation value (of a front-to-back ratio of 40 dB or more) or a technology in which a distance between the two antennas is sufficiently spaced (by 3 m or more) or an antenna direction is adjusted.
However, there may occur problems with a weight, price, wind pressure, and the like since a size of a reflector should increase in an antenna for ensuring a high isolation value.
It is difficult to sufficiently space the two antennas upon installation. There is a problem in that a price and signal loss may increase due to the cable length.
Methods of adjusting an antenna direction and increasing an isolation value have limitations since a direction in which a signal is smoothly received from the base station and a service area direction are fixed. Moreover, there is a problem in that objectivity is lack since an isolation value when a worker adjusts the antenna direction is different from that when the worker is far away from the antenna after adjustment. Since peripheral situations differ according to place where the repeater is installed, a lot of effort is required for antenna adjustment to ensure the sufficient isolation between the two antennas upon installation of the repeater.
Peripheral situations vary with time even when the sufficient isolation is ensured upon initial installation. There is a problem in that the sufficient isolation may not be ensured in general situations since the isolation frequently varies, for example, due to variation of obstacles such as a building and the like and movement of vehicles and people.